The manga covered it a few weeks ago. Kakashi, very reluctantly, killed Rin at her own insistence because she was
kidnapped by Kiri (the Land of Water) and forcefully made the Jinchuriki of the Sanbi (Three-Tailed Turtle) and planned
to be returned to Konoha to let it be unleashed and possibly destroy it, so Rin had Kakashi kill her to prevent that
from happening.